Conventionally, metallic fuel tanks have been used for automobiles, etc., but, in recent years, fuel tanks made of thermoplastic synthetic resins have been used, because they can reduce the weight of vehicles, no rust is generated therein, and they can be readily formed into desired configurations.
Automobile fuel tanks made of synthetic resins have been frequently formed by blow molding, because tubular bodies can be readily formed thereby. Upon blow molding, a parison of a molten thermoplastic synthetic resin is extruded from an upper side of a mold into a cylindrical configuration, and air is blown into the parison while holding the same with the mold, thereby forming the automobile fuel tanks.
In this blow molding method, in order to prevent the permeation of fuel oil while ensuring the strength of the fuel tank, the parison is formed to have a multi-layered structure. This multi-layered structure has at least a resin layer exhibiting shock resistance, which is adapted to ensure the strength of the fuel tank, another resin layer exhibiting barrier properties, which is adapted to prevent the permeation of the fuel oil, and a bonding layer adapted to bond these two kinds of resin layers to each other (see Patent document 1, for example.).
In this case, a main body of the fuel tank includes an outer body layer and an inner body layer, each being composed of a high-density polyethylene (HDPE) exhibiting a strength required for the fuel tank along with fuel resistance, and an intermediate layer composed of a thermoplastic synthetic resin is provided therebetween as a barrier layer adapted to prevent the permeation of fuel (see Patent document 2, for example.).
And, in recent years, in order to improve fuel efficiency of automobiles for protecting the environment, the weight of vehicles has been reduced.
It is necessary to reduce the weight of the automobile fuel tank made of a synthetic resin, too. In this case, the thickness of an outer wall of the synthetic resin fuel tank must be decreased while ensuring a sufficient volume for accommodating fuel. But, where only the thickness of the outer wall of the fuel tank is decreased, the deformation amount of the fuel tank may be increased and the heat resistance may be lowered if the internal pressure of the fuel tank is elevated.
On the other hand, in order to make the outer wall of the fuel tank thin, the parison used upon blow molding must be formed thin, but, the thin parison may break in the blow-up step of blowing air in an interior of the parison to expand the same. Therefore, in order to prevent the parison from breaking in the blow-up step, severe adjustment of the blow molding condition is needed so that the molding of the fuel tank becomes difficult.
Under the above circumstances, various kinds of polyethylene, each exhibiting high rigidity and excellent durability, have been proposed (see Patent document 3, for example.). However, the blow molding conditions such as blow moldability, deformation-suppressing properties of the fuel tank, etc. have not been sufficiently satisfied.